Phase behavior of a PEO-PPO-PEO triblock copolymer in aqueous solutions: Two gelation mechanisms

Abstract

Phase behavior of a PEO-PPO-PEO (Pluronic P103) triblock copolymer in water is investigated using small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), dynamic light scattering (DLS) and rheology. Pluronic P103 shows apparent two gel states in different temperature regions. The first sol-to-gel transition at a lower temperature (i.e., the hard gel I state) turns out to be the hexagonal microphase as evidenced by the combined SANS and SAXS and the frequency dependence of both G' and G'' in rheology. In contrast to the hard gel I, the second sol-to-gel transition (i.e., the hard gel II state) at a higher temperature represents the block copolymer micelles in somewhat disordered state rather than the ordered state seen in the hard gel I. Moreover, turbidity change depending only on the temperature with four distinct regions is observed and the large aggregates with size larger than 5,000 nm are detected with DLS in the turbid solution region. Based upon the present study, two different gelation mechanisms for aqueous PEO-PPO-PEO triblock copolymer solutions are proposed.

Phase behavior of the Pluronic P103/water system in the dilute and semi-dilute regimes

“…7), typical of a gel, are detected. It is important to emphasize that this is the first detailed phase diagram of the P103/water system in the dilute and semidilute region that has been reported, inasmuch as Alexandridis et al [8] only examined the dilute region and Park et al [26,27] did not provide any structural details of the aggregates in the sol (L 1 ) region in the P103/water phase diagram reported by them.…”

“…Park et al studied the phase behavior of P103 in water using DLS, SAXS, SANS, and rheometry [26,27]. They reported two hard gel phases (hard gel phases I and II), a soft gel, and a sol region.…”

“…At higher concentrations and temperatures these copolymers form a variety of lyotropic liquid crystals [24,25]. The phase diagrams of aqueous solutions of Pluronics have been obtained by dynamic light scattering (DLS) [12][13][14]26], static light scattering (SLS) [12,13,15], small-angle neutron scattering (SANS) [3,15,17,18,26,27], small-angle X-ray scattering (SAXS), and 1 H NMR spectroscopy [28], ultrasonic speed measurements and densimetry [29][30][31], differential scanning calorimetry (DSC), low shear viscometry, and light transmission experiments [29] as well as dynamic-mechanical rheometry [3,[12][13][14][15]26,27]. The phase behavior of Pluronics has been also predicted using both continuum and lattice mean-field theories [32].…”

Micellization and gelation of PEO-PPO-PEO binary mixture with non-identical PPO block lengths in aqueous solution

“…In fact, pure L64 at 25% does not form gel at any temperature, while pure F127 has only one gel phase. The presence of two different gel regions separated by a liquid or soft gel phase was reported previously only for very few pure Pluronics with relatively short PEO block lengths such as P103 [34,35], P94 [36] and P85 [10,36]. The second gel of P85 results from hexagonal packing of cylindrical micelles [36e38], while that of P103 is attributed to a disordered macrophase (turbid) driven by strong hydrophobic attraction between the micelles [34,35].…”

Temperature and concentration effects on supramolecular aggregation and phase behavior for poly(propylene oxide)–b-poly(ethylene oxide)–b-poly(propylene oxide) copolymers of different concentration in aqueous mixtures, 2

“…Aggregates formed by Pluronics consist of a hydrophobic core formed by PPO blocks and of a hydrophilic shell composed by the PEO chains. Increasing the polymer concentration a reversible sol-gel transition occurs [45,46]; many mechanisms have been proposed to explain this transition [47,48]. Sol-gel transition concentration, as well as critical micelle concentration (c.m.c.)…”